Angelina Razafitianamaharavo

Structural–chemical disorder of manganese dioxides

Relationships between lattice parameters of manganese dioxides and their surface properties at the solid-aqueous solution interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered MD samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional area of water molecules adsorbed in the first monolayer is positively correlated to Pr. Titration of the surface charge of the MD series evidenced a positive linear relationship between the PZC and Pr (Pr=0, Tw=0, PZC=1 for ramsdellite; Pr=1, Tw=0, PZC=7.3 for pyrolusite; gamma-MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values). The rate of microtwinning appeared as a secondary factor for the increase of the PZC. The above correlations are explained by the chemical defects at the origin of the structural disorder, respectively Mn(3+)/Mn4+ substitution for Pr and Mn vacancies for Tw, which result in proton affinity and thus in increased PZC. The experimental results are compared with data collected in the literature for manganese dioxides as well as for dioxides of transition elements with tetragonal structure.

Berthierine-like mineral formation and stability during the interaction of kaolinite with metallic iron at 90 °C under anoxic and oxic conditions

Abstract The interaction between metallic iron and kaolinite was studied in conditions relevant to those that may be encountered in a high-level radioactive waste disposal facility in geological formation. Experiments were carried out under anoxic atmosphere at 90 °C and in chloride solutions to simulate conditions close to disposal facilities. KGa-2 kaolinite was put in contact with powdered metallic iron in batch experiments for durations of 1, 3, and 9 months. Solutions extracted from the end-products were analyzed (pH, Eh, conductivity, and cation concentrations). End-products were characterized by a set of chemical (oxide analyses, CEC, EDXS) and mineralogical techniques (SEM, TEM, XRD, and FTIR), textural analyses (nitrogen adsorption and low-pressure argon adsorption), XPS, and Mössbauer spectroscopy. In another set of experiments the system was changed from anoxic to oxic conditions to evaluate the stability of the system in the presence of O2. The interaction between metallic iron and kaolinite led to a fast initial reaction as major modifications took place during the first month. The partial oxidation of metallic iron resulted in a pH increase and negative Eh values. Iron was not found in solution but in two new Fe-rich phases: magnetite in very low amounts and a Fe-rich clay phase, belonging to the berthierine family. The Si and Al of the berthierine are derived from the partial alkaline dissolution of kaolinite, mostly along edge faces. TEM-EDXS local analyses showed that the composition of resulting particles consisted in mixtures of berthierine and kaolinite layers. Clay particles became thicker with the epitaxial growth of berthierine layers on the basal surfaces of pristine kaolinite. Neoformed berthierine was not stable in the presence of O2 at 90 °C. Berthierine layers dissolved, iron was mobilized to form iron oxides and oxyhydroxides while kaolinite layers recrystallized from released Al and Si.

Structural–chemical disorder of manganese dioxides: II. Influence on textural properties

Relationships between structural parameters of MnO2 and their surface properties at the solid-gas interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered gamma-MnO2 samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth in the ramsdellite network) and Tw (rate of microtwinning). Analysis of the N2 adsorption isotherm evidenced positive correlations between specific surface area and Tw for gamma-MnO2 only and between the energetic constant C and (1-Pr). No microporosity is evidenced. Water adsorption isotherms evidenced the dependence of the H2O monolayer volume on Tw and showed a positive correlation between the cross-section area of water molecules adsorbed in the first monolayer and Pr, ranging from 13.5 A2 for Pr=1 to 6.3 A2 for Pr=0.2 (12 sites/nm2). Energetic heterogeneity is quantified from Ar and N2 low-pressure adsorption isotherms with the DIS procedure and correlated with H2O adsorption. High-energy adsorption domains are quantified and assigned to the different crystal faces: (110) faces with a common 1 x 1 octahedra layer of pyrolusite and ramsdellite and the (001) face of ramsdellite with 2 x 2 octahedra on which channels and plateaus are differentiated. The specific surface area ratio of ramsdellite high-energy sites to total ramsdellite content is shown to depend on Tw. The dependence on microtwinning of low cross-sectional area of N2 and much lower cross-sectional of residual H2O molecules leads us to assume that their adsorption sites on grain boundaries are represented by the twin planes between the structured nanocrystals generated by oxygen evolution during MD synthesis.

Physicochemical properties of talc ore from Pout-Kelle and Memel deposits (central Cameroon)

Abstract Physicochemical properties of representative samples from talc deposits discovered at Pout-Kelle and Memel in Cameroon have been investigated using a variety of techniques. The data allow the study of the mineral crystal-chemistry, chemical and modal compositions, grain size distribution, textures and surface heterogeneity, and morphometric characteristics. As a result, we found that talc (platy or round, Fe-rich) prevails (65−90%) over Cr-chlorite (7−26%), halloysite, chromite, rutile, brucite and magnesite. Chemically, talc ores are made up of SiO2, MgO, Fe2O3 and minor Al2O3. Mode values range from 50 to 55 μm at Memel and from 30 to 90 μm at Pout-Kelle. Specific surface areas measured by BET and t-plot methods range from 1.3 to 2.5 m2/g. The large values of morphometric characteristics are indicative of high crystallinity and platiness. Potential uses of these talc ores in the rubber, plastics and paper industries require beneficiation processes. Interestingly, the Memel deposit and most zones of the Pout-Kelle deposit are free of needle-shaped crystals, an advantage for environmental safety.

Characterization for industrial applications of clays from Lembo deposit, Mount Bana (Cameroon)

Abstract The Lembo clay deposit occurs on orthogneiss, but it also comprises clays with lithorelicts of volcanic rocks. In this study, ten samples from two sites were investigated. The mineralogical compositions consisted of kaolinite + halloysite + illite + quartz + hematite + anatase ± rutile ± orthoclase ± sanidine ± magnetite ± maghemite ± goethite ± Ba-Al-Fe-phosphates ± carbonates ± sulphates. Kaolinite-halloysite and quartz are the prevailing minerals. Some volcanic-derived clays contain Fe-rich kaolinite-halloysite (9.6-14.1 wt.% Fe2O3). The chemical compositions, colours and specific surface area (SSA) measurements reveal two groups of clays: one with a positive whiteness index (IB), small SSA and small Fe content, and the other showing a large SSA, negative IB and relatively large Fe contents. The compositions of the first group are close to those of clays from the Mayouom deposit (Cameroon), and from some European commercial kaolins used in ceramics. Relatively Fe-rich clay materials may conform to most formulations of earthen bricks. As a whole, the Lembo clay deposit comprises various compositions of kaolinitic clays, which may yield the opportunity for extensive application in ceramics.

Interaction of Fe(III) and Al(III) during hydroxylation by forced hydrolysis: The nature of Al–Fe oxyhydroxy co-precipitates

Al-Fe oxyhydroxy co-precipitates were synthesised by forced hydrolysis of Fe(NO3)3·9H2O and Al(NO3)3·9H2O solutions. Solids containing 0, 1, 10, 25, 50, 75, 90 mol% Al were characterised for composition, texture, mineral structure and local atomic environment. Cation substitution in the hydrous oxides was not observed. The solids consisted of hydrous ferric oxide (HFO) nuclei surrounded by Al-hydroxide. Below 50 mol% Al, unit particle size, high specific surface area and microporosity of HFO were preserved. Al-K-edge XANES showed ordered arrangements of Al-octahedra suggesting that some Al bound specifically to HFO surface sites. Above 50 mol%, Al precipitated in multiple layers around the nuclei and also as individual entities. The HFO nuclei exhibited the general characteristics of a 2-line ferrihydrite. However, as Al increased, the structure was slightly modified. While the symmetry of the FeO6 octahedra improved, the number of Fe-Fe linkages decreased, suggesting less polymerisation. An organisation of the Fe-octahedra, faintly resembling akaganeite, was expressed in Raman spectra. These changes in HFO structure were attributed to hindrance in the progressive olation/oxolation of the primary Fe-hydroxypolymers, caused by Al bound to the surface of nuclei. The presence of nitrate is suspected to have favoured the structural changes.

Surface modification of TiO2 nanoparticles with AHAPS aminosilane: distinction between physisorption and chemisorption

This paper addresses the surface modification of TiO2 nanoparticles with n-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPS) using various initial aminosilane concentrations. The main objective of this article is to show experimentally the importance of the physisorption during the grafting process. The distinction between chemisorbed and physisorbed aminosilane molecules on TiO2 is thoroughly analyzed. The surface of bare and modified TiO2 particles has been characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) to gain a better understanding of the adsorption mechanism of AHAPS on TiO2. Quantitative information on surface energy of TiO2, in terms of adsorption energy sites and heterogeneity, has been investigated by quasi-equilibrium low-pressure adsorption technique using nitrogen and argon as probe molecules. The FTIR and XPS data are combined to estimate and discuss the chemisorbed and physisorbed contribution. The results demonstrate that both physisorption and chemisorption occurs but they display a different behavior. The physisorbed amounts are much higher than the chemisorbed amounts. This shows that the main part of the adsorbed layer is composed of physisorbed molecules. The physisorbed uptake depends highly on the AHAPS concentration while the chemisorbed amount remains constant. Quasi-equilibrium Ar derivative adsorption isotherms reveal that the AHAPS molecules are mostly located on the {101} and {001} faces of titania and that the two faces display the same reactivity toward AHAPS sorption. Nitrogen adsorption experiments show that the sorption takes place on the three polar surface sites of high energy. The molecules are chemisorbed onto the site displaying the highest energy while they are physisorbed on the two lower energy sites.

REACTIVITY OF CALLOVO-OXFORDIAN CLAYSTONE AND ITS CLAY FRACTION WITH METALLIC IRON: ROLE OF NON-CLAY MINERALS IN THE INTERACTION MECHANISM

In order to better understand the possible interactions between steel canisters and a claystone host rock, in this case the Callovo-Oxfordian rock (COx), the present study investigated in detail, under conditions relevant to high-level radioactive waste repositories (anoxic conditions, temperature of 90°C), the reactions between metallic iron and: (1) COx; (2) the clay fraction extracted from COx (CF); and (3) mixtures of CF with quartz, calcite, or pyrite. Batch experiments were then carried out in the presence of NaCl-CaCl2 background electrolyte, for durations of 1, 3, and 9 months. Solid and liquid end-products were characterized by a combination of techniques including liquid analyses, transmission and scanning electron microscopies, X-ray diffraction, N2 adsorption at 77 K, and Mössbauer spectroscopy. The interaction between CF and metallic iron appeared to proceed by means of pathways similar to those illustrated in previous studies on interactions between metallic iron and purified clays. In spite of the many similarities with previous studies, significant differences were observed between the behavior of COx and CF, particularly in terms of pH and Eh evolution, iron consumption, chemical composition of the neoformed particles, and textural evolution. Such differences demonstrate the important role played by non-clay minerals in reaction pathways. The addition of carbonates or pyrite to CF did not lead to significant change in reactivity. In contrast, under the conditions used in the present study, i.e. for relatively low iron:clay ratios, the presence of quartz strongly influenced reaction pathways. In the presence of quartz, magnetite was observed only in trace abundances whereas the amounts of magnetite were significant in experiments without quartz. Furthermore, filamentous serpentine particles with a small Al:Si ratio appeared which could develop from an FeSiAl gel that only forms in the presence of quartz. Considering that most clay rocks currently being considered for radioactive waste disposal contain significant amounts of quartz, the results obtained in the present study may be of significant interest for predicting the long-term behavior of clay barriers in such sites.

Mineralogical and physico-chemical characteristics of Cameroonian smectitic clays after treatment with weakly sulfuric acid

Abstract Smectitic clays from the Sabga and Bana areas, western Cameroon were treated with sulfuric-acid solutions of various concentrations - 0.5, 0.7, 1.0 and 4.0 N - at 80°C for 2 h. The mineralogical, physicochemical and morphological characteristics of the samples treated were analysed using several techniques. The sulfuric acid caused structural modification of the dioctahedral smectite. The accessory minerals such as cristobalite, quartz, feldspars and anatase remained unaltered by the acid attack. The supernatant solutions after acid treatment contain Mg, Ca, K, Na, Si, Al and Fe as a result of partial dissolution of octahedral and tetrahedral cations. The activated clay samples exhibited a smaller cation exchange capacity (CEC) and the specific surface area (SSA) increased with increasing concentration of sulfuric acid, ranging from 65 to 134 m2/g for the sample from Bana and from 74 to 84 m2/g for the sample from Sabga. The different SSA values were affected by the relative abundance of smectite and cristobalite (SiO2/Al2O3 = 2.2 and 6.5, respectively, from Bana and Sabga). The activated clays from Bana and Sabga displayed interesting physicochemical and textural properties and can be considered as promising adsorbents for the bleaching of vegetable oils.

STUDY OF LOW-PRESSURE ARGON ADSORPTION ON SYNTHETIC NONTRONITE: IMPLICATIONS FOR SMECTITE CRYSTAL GROWTH

Because relatively little information about the crystal-growth process of smectite is available, the process was assessed here by studying the size and shape of nontronite particles synthesized at six different temperatures from 75 to 150°C over a period of 4 weeks.The morphology of nontronites was studied using low-pressure isotherms of argon adsorption at 77 K, a method which enables the measurement of the basal and edge surface areas of the nontronite particles and of their mean diameter and thickness. During the crystal growth of nontronite, the mean particle length increased whereas their thickness (and the number of stacked layers) did not vary significantly.A specific two-dimensional crystal-growth process was observed for smectite via the lateral extension of the layers. This process also appears to occur during the growth of neoformed natural smectite.